1 1 Objectives of Chapter 22  To introduce the principles and mechanisms by which corrosion and wear occur under different conditions. This includes the aqueous corrosion of metals, the oxidation of metals, the corrosion of ceramics, and the degradation of polymers.  To give summary of different technologies that are used to prevent or minimize corrosion and associated problems.

2 2 Chapter Outline  22.1 Chemical Corrosion  22.2 Electrochemical Corrosion  22.3 The Electrode Potential in Electrochemical Cells  22.4 The Corrosion Current and Polarization  22.5 Types of Electrochemical Corrosion  22.6 Protection Against Electrochemical Corrosion  22.7 Microbial Degradation and Biodegradable Polymers  22.8 Oxidation and Other Gas Reactions  22.9 Wear and Erosion

3 3 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.2 Photomicrograph of a copper deposit in brass, showing the effect of dezincification (x50).

4 4 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.4 The anode and cathode reactions in typical electrolytic corrosion cells: (a) the hydrogen electrode, (b) the oxygen electrode, and (c) the water electrode.

5 5  Electrode potential - Related to the tendency of a material to corrode. The potential is the voltage produced between the material and a standard electrode.  emf series - The arrangement of elements according to their electrode potential, or their tendency to corrode.  Nernst equation - The relationship that describes the effect of electrolyte concentration on the electrode potential in an electrochemical cell.  Faraday’s equation - The relationship that describes the rate at which corrosion or plating occurs in an electrochemical cell. Section 22.3 The Electrode Potential in Electrochemical Cells

6 6 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.5 The half-cell used to measured the electrode potential of copper under standard conditions. The electrode potential of copper is the potential difference between it and the standard hydrogen electrode in an open circuit. Since E 0 is great than zero, copper is cathodic compared with the hydrogen electrode.

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8 8  Intergranular corrosion - Corrosion at grain boundaries because grain boundary segregation or precipitation produces local galvanic cells.  Stress corrosion - Deterioration of a material in which an applied stress accelerates the rate of corrosion.  Oxygen starvation - In the concentration cell, low- oxygen regions of the electrolyte cause the underlying material to behave as the anode and to corrode.  Crevice corrosion - A special concentration cell in which corrosion occurs in crevices because of the low concentration of oxygen. Section 22.5 Types of Electrochemical Corrosion

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10 A brass fitting used in a marine application is joined by soldering with lead-tin solder. Will the brass or the solder corrode? Example 22.5 SOLUTION From the galvanic series, we find that all of the copper-based alloys are more cathodic than a 50% Pb-50% Sn solder. Thus, the solder is the anode and corrodes. In a similar manner, the corrosion of solder can contaminate water in freshwater plumbing systems with lead. Example 22.5 Corrosion of a Soldered Brass Fitting

11 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.6 Example of microgalvanic cells in two-phase alloys: (a) In steel, ferrite is anodic to cementite. (b) In austenitic stainless steel, precipitation of chromium carbide makes the low Cr austenite in the grain boundaries anodic.

12 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.7 Photomicrograph of intergranular corrosion in a zinc die casting. Segregation of impurities to the grain boundaries produces microgalvanic corrosion cells (x50).

13 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.8 Examples of stress cells. (a) Cold work required to bend a steel bar introduces high residual stresses at the bend, which then is anodic and corrodes. (b) Because grain boundaries have a high energy, they are anodic and corrode.

14 A cold-drawn steel wire is formed into a nail by additional deformation, producing the point at one end and the head at the other. Where will the most severe corrosion of the nail occur? Example 22.6 SOLUTION Since the head and point have been cold-worked an additional amount compared with the shank of the nail, the head and point serve as anodes and corrode most rapidly. Example 22.6 Corrosion of Cold-Drawn Steel

15 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 22.9 Concentration cells: (a) Corrosion occurs beneath a water droplet on a steel plate due to low oxygen concentration in the water. (b) Corrosion occurs at the tip of a crevice because of limited access to oxygen.

16 Two pieces of steel are joined mechanically by crimping the edges. Why would this be a bad idea if the steel is then exposed to water? If the water contains salt, would corrosion be affected? Example 22.7 SOLUTION By crimping the steel edges, we produce a crevice. The region in the crevice is exposed to less air and moisture, so it behaves as the anode in a concentration cell. The steel in the crevice corrodes. Salt in the water increases the conductivity of the water, permitting electrical charge to be transferred at a more rapid rate. This causes a higher current density and, thus, faster corrosion due to less resistance polarization. Example 22.7 Corrosion of Crimped Steel

17 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure (a) Bacterial cells growing in a colony (x2700). (b) Formation of a tubercule and a pit under a biological colony.

18  Inhibitors - Additions to the electrolyte that preferentially migrate to the anode or cathode, cause polarization, and reduce the rate of corrosion.  Sacrificial anode - Cathodic protection by which a more anodic material is connected electrically to the material to be protected. The anode corrodes to protect the desired material.  Passivation - Producing strong anodic polarization by causing a protective coating to form on the anode surface and to thereby interrupt the electric circuit. Section 22.6 Protection Against Electrochemical Corrosion

19 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure Alternative methods for joining two pieces of steel: (a) Fasteners may produce a concentration cell, (b) brazing or soldering may produce a composition cell, and (c) welding with a filler metal that matches the base metal may avoid the formation of galvanic cells (for Example 22.8)

20 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure Zinc-plated steel and tin-plated steel are protected differently. Zinc protects steel even when the coating is scratched, since zinc is anodic to steel. Tin does not protect steel when the coating is disrupted, since steel is anodic with respect to tin.

21 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure Cathodic protection of a buried steel pipeline: (a) A sacrificial magnesium anode assures that the galvanic cell makes the pipeline the cathode. (b) An impressed voltage between a scrap iron auxiliary anode and the pipeline assures that the pipeline is the cathode.

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23  Simple polymers (such as polyethylene, polypropylene, and polystyrene), high-molecular-weight polymers, crystalline polymers, and thermosets are relatively immune to attack.  However, certain polymers—including polyesters, polyurethanes, cellulosics, and plasticized polyvinyl chloride (which contains additives that reduce the degree of polymerization)—are particularly vulnerable to microbial degradation. Section 22.7 Microbial Degradation and Biodegradable Polymers

24  Oxidation - Reaction of a metal with oxygen to produce a metallic oxide. This normally occurs most rapidly at high temperatures.  Pilling-Bedworth ratio - Describes the type of oxide film that forms on a metal surface during oxidation. Section 22.8 Oxidation and Other Gas Reactions

25  Adhesive wear - Removal of material from surfaces of moving equipment by momentary local bonding, then bond fracture, at the surfaces.  Abrasive wear - Removal of material from surfaces by the cutting action of particles.  Cavitation - Erosion of a material surface by the pressures produced when a gas bubble collapses within a moving liquid.  Liquid impingement - Erosion of a material caused by the impact of liquid droplets carried by a gas stream. Section 22.9 Wear and Erosion

26 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The asperities on two rough surfaces may initially be bonded. A sufficient force breaks the bonds and the surfaces slide. As they slide, asperities may be fractured, wearing away the surfaces and producing debris.

27 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure Abrasive wear, caused by either trapped or free- flying abrasives, produces troughs in the material, piling up asperities that may fracture into debris.

28 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure Two steel sheets joined by an aluminum rivet (for Problem 22.25).